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Constraining Planet Structure and Composition from Stellar Chemistry: Trends in Different Stellar Populations N Astronomy & Astrophysics manuscript no. santos˙31359 c ESO 2017 November 3, 2017 Constraining planet structure and composition from stellar chemistry: trends in different stellar populations N. C. Santos1;2, V. Adibekyan1, C. Dorn3, C. Mordasini4, L. Noack5;6, S. C. C. Barros1, E. Delgado-Mena1, O. Demangeon1, J. Faria1;2, G. Israelian7;8, and S. G. Sousa1 1 Instituto de Astrof´ısica e Cienciasˆ do Espac¸o, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal 2 Departamento de F´ısica e Astronomia, Faculdade de Ciencias,ˆ Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal 3 University of Zurich, Institut of computational sciences, Winterthurerstrasse 190, CH -8057 Zurich¨ 4 Physikalisches Institut, University of Bern, Gesellschaftsstrasse 6, CH-3012 Bern, Switzerland 5 Department of Reference Systems and Geodynamics, Royal Observatory of Belgium (ROB), Avenue Circulaire 3,1180 Brussels, Belgium 6 Institute of Geological Sciences, Freie Universitat¨ Berlin, Malteserstr. 74-100, 12249 Berlin, Germany 7 Instituto de Astrof´ısica de Canarias, C/V´ıa Lactea´ s/n, 38205 La Laguna, Tenerife, Spain 8 Universidad de La Laguna, Dept. Astrof´ısica, E-38206 La Laguna, Tenerife, Spain Received date / Accepted date Abstract Context. The chemical composition of stars that have orbiting planets provides important clues about the frequency, architecture, and composition of exoplanet systems. Aims. We explore the possibility that stars from different galactic populations that have different intrinsic abundance ratios may produce planets with a different overall composition. Methods. We compiled abundances for Fe, O, C, Mg, and Si in a large sample of solar neighbourhood stars that belong to different galactic populations. We then used a simple stoichiometric model to predict the expected iron-to-silicate mass fraction and water mass fraction of the planet building blocks, as well as the summed mass percentage of all heavy elements in the disc. Results. Assuming that overall the chemical composition of the planet building blocks will be reflected in the composition of the formed planets, we show that according to our model, discs around stars from different galactic populations, as well as around stars from different regions in the Galaxy, are expected to form rocky planets with significantly different iron-to-silicate mass fractions. The available water mass fraction also changes significantly from one galactic population to another. Conclusions. The results may be used to set constraints for models of planet formation and chemical composition. Furthermore, the results may have impact on our understanding of the frequency of planets in the Galaxy, as well as on the existence of conditions for habitability. Key words. (Stars:) Planetary systems, Planets and satellites: composition, Techniques: spectroscopy, Stars: abundances ————————————————- up to ∼+0.5 dex. Thick-disc stars typically have lower metal- licities than their thin-disc counterparts. More specifically, they present higher values of α-element1 abundances (for a recent pa- 1. Introduction per, see Adibekyan et al. 2013a). Finally, halo stars are usually objects of lower metallicity, which also often present α element The study of stars hosting planets is providing a huge amount enhancement. They are commonly identified using dynamical of information about the processes of planetary formation and approaches (Bensby et al. 2003a). The kinematical and chem- evolution (see e.g. Mayor et al. 2014). The dependence on the ical properties (in particular the abundance ratios) of these three frequency of planets with the stellar metallicity and mass (e.g. populations reflect their origin, age, and the galactic formation Santos et al. 2004; Fischer & Valenti 2005; Johnson et al. 2007; process (e.g. Haywood et al. 2013a); see Appendix A for more arXiv:1711.00777v1 [astro-ph.EP] 2 Nov 2017 Sousa et al. 2011b; Buchhave et al. 2012), for example, has details. been suggested as strong evidence in favor of the hypothesis of the core-accretion model as the dominant giant planet formation Recent studies suggest that the abundances of specific chem- process (e.g. Mordasini et al. 2012a). The architecture of plane- ical species in the stellar photosphere may give clues about the tary systems and its dependence on the metal content of the stars internal structure and composition of the planets. This is true further provides indications about the processes involved in the both for giant planets (Guillot et al. 2006; Fortney et al. 2007) planet migration (e.g. Dawson & Murray-Clay 2013; Adibekyan and for their rocky counterparts (e.g. Bond et al. 2010; Delgado et al. 2013b; Beauge´ & Nesvorny´ 2013). Mena et al. 2010; Dorn et al. 2015; Thiabaud et al. 2015; Santos The stars we observe in the solar neighbourhood can be di- et al. 2015; Dorn et al. 2017). Stars from different galactic pop- vided into three galactic populations: the thin-disc, the thick- disc, and the halo population. Most stars are members of the 1 Elements for which the most abundant isotopes are integer multi- younger thin-disc component, ranging in [Fe/H] from ∼−0:8 ples of 4, the mass of a helium nucleus (α particle) 1 N. C. Santos et al.: Constraining planet structure and composition from stellar chemistry: trends in different stellar populations ulations may thus present different planet frequencies, and the [Fe/H]¡−0:2 dex) and its α-rich metal-rich counterpart (20 stars planets orbiting them may present different composition trends with [Fe/H]≥ −0:2 dex), hereafter called hαmr. Finally, 3 stars (e.g. Haywood 2008; Adibekyan et al. 2012a; Frank et al. 2014; in our sample were identified as belonging to the galactic halo Adibekyan et al. 2015, 2016a). following the kinematic criteria of Bensby et al. (2003a). They In this context, the present paper investigates whether stars were treated separately since halo stars in the solar neighbour- that come from different galactic populations and have differ- hood are also known to be mainly rich in α elements (but see ent chemical composition are expected to form planet building Nissen & Schuster 2010). blocks (or planets) with different compositions. In Sect. 2 we present the data selected for this study, including detailed chem- ical abundances for several elements. In Sect. 3 we then describe 3. Model our model, applying it to the stars in different galactic popula- tions in Sect. 4. Finally, in Sect. 5 we discuss our results in face The model we used here is the same as was used in Santos of the expected planet populations in the galaxy, including the et al. (2015). In brief, it makes use of the abundances of the prospects for life-bearing worlds. rock-forming elements Fe, Si, Mg, C, and O, together with H and He, and assumes that these are the most relevant to con- trol the species expected from equilibrium condensation models (Lodders 2003; Seager et al. 2007), such as H2, He, H2O, CH4, 2. Data 3 Fe, MgSiO3, Mg2SiO4, and SiO2 . In other words, in our model To explore the effect of different initial stellar abundances on we only include the mineral phases of the main rock-forming el- the planet composition, we need to define a sample of stars for ements that dominate the crust, the upper and lower mantle, and which precise abundances have been determined. As we show the core of an Earth-like planet interior (see e.g. McDounough below, our model needs abundance values for Fe, Si, Mg, C, and & Sun 1995; Sotin et al. 2007). A simplified model for the ex- O as input. To build this sample, we started from the work of pected mass fractions of different compounds using these species Adibekyan et al. (2012b). This study, based on spectra with high is thus a reasonable approach. In this case, the molecular abun- signal-to-noise (S/N) ratios and high resolution obtained with dances and therefore the mass fraction can be found from the the HARPS spectrograph, provides abundances of Fe, Si, and atomic abundances with simple stoichiometry, as discussed in Mg for 1111 stars in a volume-limited sample. Oxygen abun- Santos et al. (2015); see also Bond et al. (2010); Thiabaud et al. dances were then added from the study of Bertran de Lis et al. (2015); Unterborn & Panero (2016). (2015). The values derived using the 6158 Å oxygen line were We note that no star in our sample has values of Mg/Si¿2, in preferred, as these have been shown by the authors to be more which case, Si would be incorporated in olivine and the remain- precise. Finally, for C we used the carbon values recently de- ing Mg would enter in other minerals, mostly oxides. Our simple rived by Suarez-Andr´ es´ et al. (2017). All these abundances were model does not take these cases into consideration. Moreover, derived based on same set of uniform stellar atmospheric param- ten stars were found to have C/O ratios above 0.8: above this eters (namely Te f f and log g, from Sousa et al. 2008, 2011b,a). value, the mineralogy is expected to be significantly different All abundances listed in these papers (as well as the stel- (Bond et al. 2010), with carbides forming instead of the sili- lar parameter analysis) were computed relative to the Sun. cates; planet building blocks would then be strongly enriched They were transformed into absolute abundances assuming the in carbon. Since only one star was found with C/O above 1 solar composition as given in Asplund et al. (2009) for Fe (C/O=1.14), and given the typical (high) errors in the derivation (log =7.50), Mg (log =7.60), and Si (log =7.51), and as given of abundances for these species (see Bertran de Lis et al.
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